The spread between the two browsers for the fourth week of the month was 67% larger than during the third week, hinting that Chrome continues to gain momentum in the share race.

Other browsers remained steady. Through May 28, Mozilla’s Firefox accounted for 25.5% of all browsers used worldwide, while Apple‘s Safari and Opera Software’s Opera logged in at 7.1% and 1.8%, respectively.

But StatCounter’s numbers are contentious in some quarters.

Rival metrics firm Net Applications, for example, cites data that shows Chrome far behind IE, with April’s numbers spotting Chrome at 18.9% and IE at 54.1%, or almost three times larger. Net Applications does not make its daily share data available to the public, so a direct comparison with StatCounter’s numbers through Monday was not possible.

Although both companies discard Chrome’s pre-rendered pages — those that the browser loads in the background in case the user decides to view them — their methodologies differ significantly. For one thing, StatCounter tallies page views while Net Applications counts unique visitors.

More importantly, Net Applications — but not StatCounter — weights its data by country to account for the lack of Western insight into browsing habits in places like China, where IE is the overwhelming favorite. The result is that Net Applications’ numbers for IE are always much larger, and Chrome’s much smaller, than StatCounter’s.

The two companies’ numbers for Firefox, Safari and Opera are typically in the same ballpark. In April, 4.7 percentage points separated the numbers for Firefox, 2.3 points for Safari and just one-tenth of a percentage point for Opera.

The differences between their shares for IE and Chrome, however, were much larger: 13.3 percentage points for Chrome and a whopping 20 points for IE.

Those variances have been obvious of late. In the early months of 2012, Net Applications revealed a rebound of IE and a halt to Chrome’s usual growth. By Net Applications’ calculations, the turnaround has been IE’s most significant and longest-sustained since the browser began shedding share last decade, first to Firefox, then to Chrome.

Meanwhile, StatCounter has spotted no sign of an IE recovery, and has said IE’s share fell and Chrome’s climbed in each of the first four months of the year.